This invention relates to hydraulic lash adjusters for taking up slack in a valve train, and to valve train assemblies which incorporate hydraulic lash adjusters.
A typical structure of this type is shown schematically in FIG. 1. The valve train assembly 2 comprises a rocker arm 4 and a hydraulic lash adjuster 6. One end 8 of the rocker arm 4 engages the stem 10 of a valve 11. The other end 12 of the rocker arm is mounted for pivotal movement on the lash adjuster 6.
The rocker arm 4 is provided with a roller 14 mounted on an axle 16 carried by the rocker arm 4.
A cam 18 mounted on a cam shaft 15 has a lobe 17 which can engage the roller 14 and thus pivot the rocker arm 4 anti-clockwise as shown in the drawing. This depresses the valve stem 10 against the force of a valve spring (not shown) and thus opens the valve. As the cam continues to rotate, and the base circle 19 of the cam profile again engages the roller 14, the valve spring returns the valve and the rocker arm 4 to the position shown in FIG. 1.
As is well known, a hydraulic lash adjuster has an oil-containing chamber and a spring arranged to enlarge the chamber and thus extend the lash adjuster. Oil flows into the chamber via a one-way valve, but can escape the chamber only slowly, for example via closely-spaced leakdown surfaces.
Accordingly, the lash adjuster 6 of FIG. 1 can extend to accommodate any slack in the valve train assembly, such as between the cam 18 and the roller 14. After it is extended, however, the oil-filled chamber provides sufficient support for the pivoting movement of the rocker arm 4.
It is important for the base circle 19 of the cam 18 to be concentric with respect to the axis of rotation of the cam shaft 15. Any slight eccentricity (xe2x80x9crun-outxe2x80x9d) could cause the valve to close later than it should, or open during the movement of the base circle past the roller 14. The cam 18 is often formed by sintering and may not have, in its initial state, particularly accurate dimensions. Accordingly, it is conventional, before assembly, to grind either the outer surface, including the base circle 19, of the cam 18, or to grind the inner diameter which is fitted to the cam shaft 15, to ensure accurate concentricity of the base circle 19 relative to the axis of rotation of the cam shaft 15.
Although the arrangement described above works well during normal running conditions, problems can arise in certain circumstances. For example, in order to prevent problems when starting the engine from cold, it has been proposed to use a technique whereby the valves and cylinder head are caused to heat up very quickly. Referring to FIG. 2, the rapid heating of the head 20 of the valve 11 causes the head 20 to expand relative to the valve seat 21. This expansion results in the valve moving downwardly against the force of the valve spring, as shown on the right of FIG. 2. This process creates positive lash, which is accommodated by expansion of the hydraulic adjuster as the camshaft rotates. However, as the cylinder head 22 and the valve seat 21 then heat up, their expansion allows the valve 11 to move back upwardly, thus creating negative lash (which will be subsequently exacerbated due to expansion of the valve stem). This negative lash can be accommodated by shrinking of the lash adjuster. However, because the heating process is taking place rapidly, and the shortening of the lash adjuster is limited by the rate of leakage of oil from the high pressure chamber, the lash adjuster does not shorten sufficiently quickly. This problem is exacerbated because the oil is still cold and therefore viscous, thus reducing the leakage rate. This results in valves remaining open (shown in dotted lines in FIG. 1), causing starting problems.
There have been proposed lash adjusters which provide xe2x80x9clift lossxe2x80x9d, that is, which are capable of shrinking to a certain extent before the sealed high-pressure chamber prevents further movement. See for example U.S. Pat. No. 6,039,017. Thus, there is a degree of lost motion of the lash adjuster before the valve starts to open. This lost motion is recovered by a spring after the valve has closed. Using such a lash adjuster, a small degree of negative lash can be quickly accommodated by the lost motion of the lash adjuster, thus making it more certain that the valve will close.
There are also lash adjusters which incorporate a seal to prevent leakage of oil from the high-pressure chamber, and in which the chamber valve is arranged such that it is normally open (known as xe2x80x9csealed-leakdownxe2x80x9d adjusters). See U.S. Pat. No. 5,622,147. This would permit a small amount of shortening of the lash adjuster before the valve closes as a result of the hydrodynamic force of the oil flowing out of the chamber. However, the amount of lift loss produced is somewhat uncertain, and will depend significantly on oil viscosity and hence temperature, as well as other factors. Also, this form of lash adjuster can sometimes encounter problems when a hot engine is stopped with a valve partially open. The pressure of the valve spring on the lash adjuster causes the high-pressure chamber to remain sealed, so that, if the engine cools and negative lash is created, oil cannot flow out of the chamber and the lash is therefore not accommodated.
It would be desirable to provide a lash adjuster of the sealed-leakdown type in which such problems are at least mitigated.
Aspects of the present invention are set out in the accompanying claims.
In a first aspect of the invention, the high-pressure chamber is sealed by a sealing means engaging both the body of the lash adjuster and the plunger as the plunger moves inwardly, thus preventing further inward movement. The arrangement is such that as the cam turns, and returns to base circle, and the pressure on the plunger decreases, the plunger and sealing means separate, preferably assisted by a biasing means such as a leaf spring. Accordingly, the pressure in the chamber is relieved whenever the base circle of the cam is reached. Because the chamber is open, the plunger assembly can be pushed inwardly by a certain amount to guarantee valve closure before the chamber is again closed.
According to a preferred aspect of the invention, it has been perceived that use of a hydraulic lash adjuster which provides lift loss (preferably, but not necessarily, an adjuster according to the first aspect of the invention) means that the base circle radius variation of the cam no longer has to be minimized by grinding, allowing the use of net-shaped cam shaft technology instead of more expensive ground cams.